1. Field of the Invention
The present invention relates to a control device for an electric vehicle that controls electric power from a high-voltage battery to drive a drive motor and accessories.
2. Description of the Related Art
An electric vehicle is equipped with a high-voltage battery comprised of a lithium-ion rechargeable battery, nickel metal hydride rechargeable battery or the like, and a drive motor comprised of a three-phase DC motor, PM motor or the like. Electric power from the high-voltage battery (approximately 300 volts) is power converted by an inverter that is controlled by a vehicle control unit and provided to the drive motor. In addition, electric power from the high-voltage battery is stepped down to approximately 14 volts by a DC-DC converter and provided to accessories such as a power steering pump, negative pressure pump for a brake, electric motor for a wiper device and headlight.
Since the high-voltage battery drives various loads installed at the electric vehicle as described, the high-voltage battery is designed to be resistant to repeated charging and discharging. However, due to the its nature, when the remaining energy capacity (SOC) of the high-voltage battery substantially decreases and the high-voltage battery enters a deep discharge state, decreased functionality of the high-voltage battery such as decrease in charging capacity occur. Therefore, a drive state of the drive motor and accessories, i.e., remaining energy capacity of the high voltage battery is monitored so that the high-voltage battery is prevented from entering the deep discharge state.
As a technology that prevents the high-voltage battery from entering a deep discharge state and secures driving performance of the electric vehicle, a control device for an electric vehicle is known that is disclosed in, for example, Japanese Unexamined Patent Application Publication No. 1998-304503 (hereunder referred to as “Patent Document 1”) (See especially FIG. 4 of Patent Document 1). The control device for an electric vehicle described in Patent Document 1 detects a remaining energy capacity (battery voltage) of the high-voltage battery and decreases output of the drive motor according to a decrease in the remaining energy capacity. Accordingly, the high-voltage battery can be prevented from entering a deep discharge state and driving performance of an electric vehicle can be secured.
However, according to the control logic of the control device for an electric vehicle described Patent Document 1, when the remaining energy capacity of the high-voltage battery becomes low, and, for example, the electric vehicle enters a state such that it stops (driving impossible state), the accessories driven by the high-voltage battery may also stop. In this case, when the accessories stop while the electric vehicle is driving on a flat road or uphill, the electric vehicle does not exhibit an acceleration behavior and hardly gives anxiety to the driver. On the other hand, when the accessories stop while the electric vehicle is driving on a downhill, the electric vehicle exhibits an acceleration behavior but the operational performance of accessories assisting the driver's operation becomes low, which may give anxiety to the driver.